You can keep playing this mission once you've completed it. Each time you beat your time record, you receive a higher prize (in money).

Glitch

Due to a bug in the game, the initial scores of both Trial By Dirt and Test Track will have the same time results once both scores are listed in the game's stats. Once you play the missions again and beat each time, the scores will be individual as they should be. The bug can be avoided by completing Test Track before Trial By Dirt.